Fast neutron nuclear reactors generally employ a liquid metal such as sodium as a heat transfer fluid. During the operation of the reactor, this liquid sodium, which circulates in the vessel in contact with the core and with the various parts of the internal equipment of the reactor vessel, becomes laden with impurities such as oxides and hydrides, which are dissolved in the sodium, or such as solid particles in suspension.
It is therefore necessary to purify the liquid metal heat transfer fluid during the use of the reactor.
In general, use is made of complex devices permitting a part of the liquid sodium to be withdrawn, cooled and passed through a filter consisting of metallic fibers, for example of stainless steel. The impurities retained by the liquid metal precipitate preferentially on the metallic fibers of the filter if the temperature of the liquid metal is sufficiently low. In this way a cold trapping of the impurities is carried out.
Such devices permitting the cold trapping of the impurities comprise a pump for circulating the liquid metal, an economizer exchanger permitting the heat transfer between the hot unpurified metal and the cold purified metal to be ensured, a device for additional cooling of the liquid metal to be purified and a filter cartridge of metallic fibers.
Such devices can be submerged in the reaction vessel, these devices being then called integrated, or alternatively mounted outside the vessel in a secondary liquid metal loop, and these devices are then called secondary.
Such devices have been described, for example, in French Pat. Nos. 2,246,942 and 2,395,570. These devices have a complex structure in which the various elements of which they are composed are placed above each other in the axial direction of the device. The length of these assemblies is therefore relatively great and, when they are arranged vertically inside the vessel of a fast neutron nuclear reactor, their robustness with regard to external constraints, for example during an earthquake, can prove to be inadequate. These assemblies comprise numerous runs of pipework connecting their various parts.
In the case of the device described in U.S. Pat. No. 2,246,942, the filter cartridge forms a mechanical assembly comprising a central tube for circulating the sodium and a sodium overflow in its upper part.
The device described in U.S. Pat. No. 2,395,570 comprises an extremely complex sodium loop having many tubular linking components.
In the case of both devices, the coil for cooling the sodium which is to be purified is moreover incorporated in the device so that its disassembly is difficult to carry out and so that it is contaminated by the sodium which is to be purified coming from the vessel containing the reactor core.
In the event of a leak in this cooling coil, its disassembly is difficult and requires a complete decontamination operation.
The known devices of the prior art are therefore complex in their structure; this increases the costs of manufacturing and maintaining these devices which have, furthermore, a great length, numerous sections of pipework interconnecting the parts and, as a result, a resistance to external stresses, such as those due to earthquakes, which can be inadequate. These devices are also subjected to constraints of a thermal origin which can be considerable and which require the provision of connecting pipework of a special shape (lyre).